1. Field of the Invention
The present invention relates to the field of driving of backlight of liquid crystal panel, and in particular to a DC (Direct Current) boost topology circuit for backlight driving.
2. The Related Arts
An LED has advantages such as long lifespan and saving energy and thus, the LED lights are getting more popular as light sources for color LCD televisions. Since the LED lights are driven by constant current driving, in practical applications of LED, series connection is commonly adopted to connect a plurality of LED lights and driving with DC power is used.
In the state of the art, a DC-DC convertor (DC conversion) portion of backlight driving of a liquid crystal panel usually comprises a boost topology circuit to drive an LED light. As shown in FIG. 1, FIG. 1 is a schematic view showing a DC boost circuit for backlight driving of a liquid crystal panel in the state of the art. The circuit comprises a DC voltage input terminal Vin, a DC voltage output terminal Vout1, an inductor L1, a diode D1, a capacitor C1, a resistor R1, a field-effect transistor Q1, and three LED lights D3, D5, D7. Specifically, positive terminal of the DC voltage input terminal Vin is connected to an end of the inductor L1. The inductor L1 has an opposite end connected to an anode of the diode D1. Cathode of the diode D1 is connected to the DC voltage output terminal Vout1 and is also connected to an end of the capacitor C1. The capacitor C1 has another end connected to negative terminal of the DC voltage input terminal Vin. The field-effect transistor Q1 has a drain terminal connected between the inductor and the anode of the diode D1. The field-effect transistor Q1 has a source terminal connected to an end of the resistor R1 and an opposite end of the resistor R1 is grounded. The three LED lights D3, D5, D7 are connected in series and connected to the DC voltage output terminal Vout1.
In charging, the field-effect transistor Q1 serves as a conductive line and the DC voltage input terminal Vin stores energy in the inductor L1. The diode D1 prevents the capacitor from discharging to the ground. Since the input is a DC power, the electrical current of the inductor L1 is linearly increased by a predetermined ratio. This ratio is related to the size of the inductor L1. With the electrical current of the inductor L1 increasing, energy is stored in the inductor L1. In discharging, the field-effect transistor Q1 functions as an open loop. Due to the holding characteristic of electrical current of the inductor L1, the electrical current flowing through the inductor L1 does not turn immediately to 0 and is instead slowly changing from the value of being just charge to 0. Since the original circuit is open, the inductor L1 can only discharge through the circuit, meaning the inductor L1 starting to charge the capacitor C1. Under this condition, the voltage is higher than the input voltage so as to drive the LED lights.
However, in the conventional boost topology, Duty=1−Vin/Vout1, since the maximum value of Duty is limited, the maximum value of Vout1 in the topology is limited. This makes the driving capacity of Vout1 limited, meaning the maximum number of LEDs that are connected in series is limited. This makes the maximum luminance of the LED backlighting of a liquid crystal panel limited. Obviously, to provide an effective backlight driving circuit for a liquid crystal panel and to increase the maximum luminance of the backlight of the liquid crystal panel are issues that the present inventor and those devoted themselves in the industry must improve.